Hycean:
A hycean planet (/ˈhaɪʃən/ HY-shən; portmanteau of hydrogen and ocean; note that the term is not capitalized) is a particular type of exoplanet that features a liquid water ocean under a hydrogen-rich atmosphere. [Wikipedia]
I wonder how many English words are portmanteaus. Noted in “Habitable ocean world K2-18b may actually be inhospitable gas planet,” Jonathan O’Callaghan, NewScientist (9 March 2024, paywall):
In 2015, astronomers discovered a planet 110 light years away called K2-18b, which later analysis estimated to be a super-Earth or mini-Neptune about eight times the mass of our world. Astronomers in 2019 then found evidence for water vapour on the planet. Since K2-18b is thought to be in the habitable zone of its star – the region in which water can exist as a liquid on the surface of a planet – that led to suggestions it might be an ocean world. Prospects were further buoyed by 2023 research finding evidence of dimethyl sulphide, a molecule that, on Earth, is only produced by life, particularly marine phytoplankton.
But now, Nicholas Wogan at the NASA Ames Research Center in California and his colleagues have re-analysed the planet using data from the James Webb Space Telescope (JWST) and they suggest that the likelihood of the planet being a habitable ocean world is low. They say that an abundance of methane and carbon dioxide found by JWST points to the planet being a gas-rich mini-Neptune with no surface, rather than a habitable super-Earth with an ocean and sea floor, because such gases would be broken down by a chemical process called photolysis if it were an ocean or “hycean” world.
